Typically a large number of semiconductor devices are formed on wafers. The semiconductor devices are formed by repeating a number of basic operations on a wafer. The basic operations include layering, patterning, doping, and heat processing. The final semiconductor formed has many layers of material and includes as many as 10,000 or more individual transistors. Rather than make each semiconductor device individually, a number of devices are formed on a single wafer. The entire wafer is subjected to the basic operations discussed above in order to form hundreds of semiconductor chips or devices. Typically, after the semiconductor devices are formed, the semiconductor devices are tested and sorted. Next, the wafer is sliced and diced so that each individual semiconductor device is separated from the others formed on the wafer.
The individual semiconductor devices are formed on fragile material. As a result, the individual semiconductor devices are packaged, in part for physical protection. Packaging also dissipates the heat from the semiconductor and provides connections or leads between the individual chip or die and an exterior portion of the package. The leads allow for electrical connection between the chip or die and a printed circuit board or other device.
There are many different types of packages. One common package is a flip chip package which has a series of bumps or balls or leads formed in an array on a surface of a substrate. The substrate includes a number of pads, typically laid out in an array. Solder individually, or mixed with flux is deposited onto the pads and then the substrate is heated. The heat applied is sufficient to reflow the solder and melt the solder to a liquid state. The flux material is used to clean the metal pads on the die and the substrate, and the solder of any oxides that may have been formed on it. Removal of the oxides allows for good solder wetting and good joint formation. The material surrounding the solder pad typically repels or resists the liquid solder. The liquid solder, therefore, wets to the surface of the pad and the surface tension of the liquid solder causes the solder to form a ball or bump. The ball or bump shape is maintained while the solder cools. The silicon die along with the substrate and the interconnections between them is referred to as an electronic package or simply the “package”. Typically, the entire package is heated and cooled to attach the die to the substrate. In most manufacturing processes, the package is reheated and cooled a number of additional times as the part of the manufacturing process. The die or chip within the package is also reheated and cooled an additional number of times. In some packaging operations, the package is reheated to clean the package of residual flux that may be burnt and charred during the die to substrate assembly process. A liquid is typically used to clean the package of excess solder, and the package is cooled. The next step is to reheat the package again to drive off any remaining liquid that may have been absorbed by the packaging materials during the residual flux cleaning operation. The substrate is then cooled. Finally, an epoxy is used to encapsulate the solder balls between the die and the substrate. Part of the encapsulation includes placing epoxy between the die and the package. This is referred to as underfilling the package. The epoxy can be pressurized during underfill or the epoxy can be heated and capillary action used to underfill the package. The epoxy adhesive is heated so that the epoxy flows into the spaces between the die and the substrate. The substrate and die or chip are then cooled again. A lid can then be placed on the package. After the packaging process is complete, heat is then managed using heat sinks or the like with the package as it operates.
Partial encapsulation is discussed above. Another common package used for flip chips is a molded epoxy enclosure. In this type of package, the die is attached to a lead frame. Then the entire die and lead frame is placed in a mold. The lead frame and the package is then surrounded with epoxy material that has been softened and heated previously. Although the process is somewhat different, it should be noted that the die or chip is subjected to repeated heating and cooling.
When the layers in a die or chip are subjected to thermal cycling of the magnitude as from reflowing the solder, washing away excess flux, driving off liquid from the washing operation and from either underfilling the space between the chip and the substrate or molding the package around the chip or die, delamination may occur between the layers in the die or chip. The repeated thermal cycles during the chip or die attach process cause delamination of the inner layer dielectrics (ILD) on the die. The ILD is a fragile, thin film layer. When delamination occurs, the chip or die fails. The delamination may result in significant yield losses. The delamination of the ILD may not only cause current failures, but can also cause latent failures that occur after the chip has left the manufacturing site. Failure of components is never desirable and generally requires more effort when the chip is in the field and must be replaced.
The description set out herein illustrates the various embodiments of the invention and such description is not intended to be construed as limiting in any manner.